Figure 1. Straight line of holes in a corn leaf. Courtesy: Adam Varenhorst
While scouting corn fields last week we came across a few plants that had a unique pattern of holes on the leaves. The holes were all uniform in size and were in a perfectly straight horizontal line on the leaf (Figure 1). So what insect is capable of feeding so precisely? The insect in question is none other than the European corn borer. The observed defoliation pattern wasn’t from precise feeding, but rather feeding on the leaf when it was still curled in the whorl. Once the leaf expanded, the line of holes appeared. This type of defoliation is commonly referred to as shot-hole injury.
Although we don’t talk about the European corn borer much, it is still present in South Dakota. In this case, the caterpillars were feeding on a Bt hybrid corn plant, and weren’t present when scouting occured. However, for conventional corn fields without Bt toxins, European corn borer can cause serious problems.
In the absence of management strategies, each European corn borer caterpillar can cause per-plant yield losses of 2-6%. Considering that a single plant may have 1-3 caterpillars present, it is possible for yield losses as high as 12% per plant. European corn borer caterpillars feed in the whorl and tunnel into the stalk; therefore, managing them with foliar insecticides can be difficult. It is estimated that, at best, only 80% management of a population can be obtained with foliar insecticides. During this time of the season, yield loss from three caterpillars is estimated to be between 8.1-3.7% (pre-tassel-dough stage). The stalk feeding by the caterpillars reduces water and nutrient flow, and also can cause later season lodging.
To scout for European corn borer, 20 plants from five areas of the field should be examined. Plants should be checked for defoliation. If defoliation is present, the plant should then be dissected and the number of caterpillars that are present should be counted. The European corn borer caterpillar is light tan with a dark brown head (Figure 2). The University of Illinois has a worksheet that can be used to determine if an insecticide application for European corn borers would be profitable. For a list of insecticides that can be used for European corn borer management, please refer to the current edition of the South Dakota Pest Management Guide: Corn.